In an effort to recover the heat content of municipal solid waste (SW), the SW is processed to a product for burning with coal in steam raising power boilers, or burning alone in power boilers for steam and/or electricity generation. Two principal products from SW are prepared for this purpose: (i) shredded "light sort", pieces 4" to 6" in size, and (ii) extruded product, 1/2" to 2" in diameter, about 2" long. These are then fed to boilers.
To make these products it is necessary to shred the SW using one shredder, or better two in series, shredding everything from the garbage truck with the exception of white goods (refrigerators, etc.) and demolition debris (cinder blocks, etc.). This procedure was found through hard experience to have two drawbacks: (i) explosions within the shredder as the high speed hammers hit sticks of dynamite, artillery shells and land mines, etc.; (ii) high boiler ash content due to fragmenting of glass (up to 10% by weight of SW) by the high speed hammers of the shredders, the glass pieces being embedded in the paper of the shredded or extruded product. Understandably this proved undesirable due to increased ash burden on the boilers.
Recognizing these disadvantages, the step of trommeling, before shredding, was advanced. The trommel is a rotating drum, say 10 feet in diameter and 20 feet long of steel plate with about 4" diameter holes in sidewalls, with lifting bars inside, rotating at low speed. The SW, fed into this trommel, which is mounted at a small angle to the horizontal, is dispersed, carried up the sidewalls by lifters and dropped back to the bottom of the drum. Everything possible falling through the holes will pass through. This includes the greatest percent of glass items (intact), tin cans (ferrous) and like heavy items as well as some small loose pieces of paper and plastic. The lifting-dropping action inside the trommel serves to break and disperse bags of refuse as well as boxed refuse.
Separation of glass and tin cans within the trommel is aided by prompt and rapid removal of paper, corrugated board and plastic film from the machine which together will be about 50% of the weight of trommel charge. This is done conveniently by blowing air countercurrently and at predetermined and relatively high velocities through the interior of the trommel. The air currents lift out and convey paper, corrugated material, plastic film and leaves, dust and such light components of the SW. Through introduction of this air current, "light sort" is lifted out and separated from the mass of SW.
In installations using this system, the solids of the "light sort" such as paper, plastic films, etc., are separated from the conveying air in a large diameter dry cyclone. The cyclone is a vertically mounted cone with a cylinder of maximum cone diameter mounted on top. The cone/cylinder is arranged with apex or bottom of converging sidewalls looking down. Use of the trommel as here described greatly reduces or eliminates the frequency of explosions. Also, less power is used in shredding of the "light" sort. Additionally, the fuel product recovered has reduced ash, due to lower glass content. It is used in this invention to separate the air conveyed fraction "light sort" from the "heavy" sort, that falls through.
The working of the invention will be set forth in the following description. It will be readily apparent to one skilled in the art that well known art may be substituted for one or more of the parts herein shown, while substitution will not be grounds for negation.